Generally, a headphone covers the whole of a user's ear to transmit a sound to the user, and an earphone is inserted into an ear canal of a user to transmit a sound to the user.
Also, the earphone has a built-in speaker, which vibrates air by using a high-low motion generated when an electrical input signal is transferred to a sound coil of a sound panel, and therefore, converts an electric signal to a sound to transmit the sound to a user.
However, in an earphone or an headphone having a built-in speaker, it is impossible to reproduce every tone of audio-frequency bands with a constant sound pressure level SPL, and even though a satisfactory SPL is outputted from a specific band, a quality of the sound may be changed according to a size of the speaker, a material of a vibration plate or the like.
Therefore, various speaker systems such as a so-called two-way speaker, a three-way speaker, a multi-way speaker or the like are made in response to various requirements of users and used by assembling speakers such as a woofer, a midrange, a tweeter or the like, the speakers used as a sound generating means. Here, a playback audio bandwidth of each of the speakers is different.
However, in a related art two-way earphone or headphone, a woofer generating a low sound is disposed in parallel with a tweeter generating a high sound in an abscissa axis, and thus, the scheme of using the woofer and the tweeter is unsuitable for an earphone which has to be miniaturized and lightened. Therefore, an earphone adopting a balance armature type speaker that is an ultra-small speaker is widely used, the balance armature type speaker being used in a hearing aid. However, a process of manufacturing the earphone having the balance armature type speaker is complicated and there are limitations in terms of a reproduction of a low sound.
Moreover, in an earphone inserted into a user's ear to be used, because an internal space, in which a speaker as a sound generating means is built, is small, it is impossible to simultaneously build two or more speakers in the earphone, the speakers having different playback audio bandwidths. Therefore, it is difficult to meet various requirements of users who want to listen to the three-dimensional sound such as a sense of a front-and-rear direction or space sense, and there are limitations in making a high-quality earphone.